It is generally known to equip antifriction bearings with measuring devices for detecting the forces acting on the bearing. For example, DE 27 46 937 A1 shows a force measuring bearing in which strain gages are fixed in a peripheral groove in a stationary bearing outer ring and are connected to other electric resistances in an electric measuring bridge. When the antifriction elements of the bearing roll over the fixing locations of such strain gages, which change their resistance as a function of strain, a substantially sinusoidal measured signal is generated, which can be analyzed by a suitable evaluation device.
In addition to the determination of the forces acting on the antifriction bearing, there is a need for information as to whether and to what extent a component held by the bearing has a balance error. Balance errors of this type arise, for example in the case of drive shafts, entirely as a result of irregular wall thicknesses of the cylindrical shaft wall or as a result of eccentric fixing of a shaft flange to the shaft tube. Furthermore, a nonuniform welded seam can also cause a balance error on such a drive shaft. The avoidance or compensation of such balance errors is primarily of great economic significance because these act on the bearing and, depending on the magnitude of the balance error, that is to say on the level of the incorrect mass distribution, and the rotational speed, these lead more or less quickly to permanent bearing damage, which can ultimately cause the total failure of a machine.
In order to avoid balance errors of this type, the rotatable components are normally clamped into a balancing device at the end of the production process and checked there for the presence of balance errors. As soon as the location of the balance error and its magnitude has been determined, the balance error can be eliminated, for example by fitting additional masses (also called canceling masses) or by the removal of the mass causing the balance error.
In addition to the balance error induced by the production process, balance errors can also occur in rotatably mounted bodies during their use, however. For example, in a case of a drive shaft used in a dirty fabrication area, balance errors induced by operation can arise from the fact that, over the course of time, dirt accumulates at different points on the shaft surface and an unbalanced weight of the rotating masses is established. In another case, for example, as a result of an object rubbing periodically in an undesired manner on the drive shaft, over the course of time material can be removed from the surface of the drive shaft at a specific point, which likewise leads to an unbalanced weight of the rotating masses and therefore to a balance error.
The disadvantage when operation-induced balance errors occur is that these generally cannot be detected immediately and unambiguously. Instead, it is usual that such an operation-induced balance error is only detected by the failure of one of the bearings in which the body is mounted. When such bearing damage has occurred, it is often necessary for an entire system to be stopped for a bearing change, which leads to considerable production failure-induced costs.
In order to detect a balance error in a rotatably mounted component, it is known to arrange for a measured signal generated by strain gages on the bearing to run through a frequency filter, which separates a carrier frequency from a modulation frequency of the measured signal. In the process, the undistorted sinusoidal measured signal oscillation caused by the antifriction elements rolling over periodically is viewed as the carrier frequency, while the forces acting on the sensors of the bearing because of the balance error are designated the modulation frequency.
The disadvantage with this known method is that, in the event of a change in the modulation frequency, for example because of a change in the rotational speed of the component, the frequency filter also has to be readjusted accordingly with regard to its filter characteristics. This can be implemented in practice only in the case of digitally operating frequency filters, but is associated with considerable and therefore time-consuming computational effort. For this purpose, what are known as “observers”, which are based on specific mathematical functions, are often readjusted adaptively. However, with regard to the analytical method applied there, attention must be paid during the readjustment of such frequency filters, to results that are also still plausible and that can be achieved. As a rule, this is made more difficult by the fact that such digital filters have a transient response which has a detrimental influence on the speed of detection and accuracy of detection with regard to the balance error to be determined.
Another method for determining the balance error of a rotatably mounted body likewise starts from the aforementioned amplitude-modulated measured signal, in which the determination of the magnitude of the frequency response of the balance error is carried out by means of a Fourier transformation. However, since the Fourier transformation includes an averaging process, in the event of a rapid change in the rotational speed of the component, the assignment of spectral components which allow conclusions to be drawn about the balance error is difficult to carry out. In addition, the resolution of the magnitude spectrum is determined by the length of the time interval which can be used for the transformation. Measured signal analyses for determining balance errors by means of the Fourier transformation can therefore as a rule only be carried out off-line, that is to say with a time delay, because of the necessary calculation steps. This is primarily disadvantageous in the case of balance errors which arise as a result of operation, since these arise in a completely unsuspected manner and can build up quickly with a destructive effect.